1. Field of the Invention
This invention relates to optical waveguide fibers and, in particular, to methods for determining optical properties of such fibers using an optical time domain reflectometer (OTDR). More generally, the invention relates to methods for reducing the noise in an electrical signal, such as the electrical signal produced by an OTDR.
2. Description of the Prior Art
OTDRs operate by sending a short pulse of laser light down an optical waveguide fiber and observing the small fraction of light that is scattered back towards the source. Typical pulsewidths may range from 0.5 meters (5 ns) to 2000 meters (20 .mu.s).
In practice, the fiber under test is connected to the OTDR by a relatively short length of fiber (e.g., a one kilometer length of fiber) known in the art as a "pigtail." The pigtail reduces the deadzone (non-linear region) at the start of the fiber where the OTDR does not provide reliable information. To further improve performance, an index matching oil can be used at the junction between the pigtail and the fiber.
A typical OTDR trace is shown in FIG. 1 where returned power in dBs is plotted along the y-axis and distance down the fiber is plotted along the x-axis. Various features of this trace are identified by the reference numbers 1 through 9, where the number 1 shows the reflection which occurs at the junction between the OTDR and the pigtail, the number 2 shows the trace obtained from the pigtail, the number 3 shows the last point of the pigtail and the first point of fiber under test, the number 4 shows the reflection and associated deadzone produced by the junction between the pigtail and test fiber, the number 5 shows the first point after the near-end deadzone at which trace information can be examined reliably (the "fiber start"), the number 6 shows the fiber trace between the fiber start and the physical end of the fiber (the "fiber end"), the number 7 shows the fiber end, the number 8 shows the reflection which occurs at the fiber end, and the number 9 shows the inherent noise level of the OTDR trace.
OTDRs have been used to determine axial variation of many attributes of optical fibers. The principal attribute is that of attenuation, but others, such as mode field diameter variation can also be measured. As the optical fiber market has expanded, the requirements on axial uniformity have tightened. The fiber lengths that are measured have also increased. This has created a demand for increased resolution at lower returned power levels. There are several hardware approaches to satisfying this demand: increased input power, broader pulses, longer sampling time. Another approach is to provide improved procedures for dealing with the noise contained in an ODTR trace. The present invention is concerned with this latter approach.